Physics Chapter 4


Mechanical Characteristics

  • Hard
  • Difficult to Dent
  • Elastic
  • Regains original shape after a deforming force is removed
  • Brittle
  • Fractures without deforming (opposite of tough)
  • Strong
  • Large force needed to break it
  • Malleable
  • A sheet of material can be hammered or pressed into a required shape
  • Stiff
  • Difficult to stretch or bend
  • Plastic
  • Permanent change of shape under a deforming force
  • Tough
  • Deforms pleastically before breaking (opposite of brittle)
  • Ductile
  • Can be drawn intoa wire
  • Density
  • Is the mass per unit volume of a substance
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Examples of strength and toughness

Strong and Brittle (i.e. not tough)

e.g. Cast Iron, Pottery, Glass and Brick

Tough and Weak (i.e. not strong)

e.g. Plastic Cup, Rubber Tyres

Strong and Tough

e.g. Steel and Fibre Glass

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Three Classes of Materials


  • Used widely
  • Good electrical and thermal conductors
  • Are shiny and usually easy to shape and form
  • A metal can be an element or an alloy


  • Materials such as bricks, tiles, plates and cups
  • All are strong and stiff, but brittle


  • Materials that are often tough and plastic
  • Synthetic polymers are often referresd to as plastics and include nylon and polyester
  • Natural polymers include rubber
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  • Bone
  • Stiff, Strong and light-weight compared to steel
  • Paper
  • Flexible and smooth enough to write on, and it absorbs ink well
  • Concrete
  • Stiff and strong in compression, capable of supporting large loads
  • Glass Fibre Panels
  • Strong, Stiff and much less dense than steel panels
  • Wood
  • more flexible and less dense than bone, but is not very strong
  • Leather
  • tough, strong and flexible
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Forces that Deform

Tension (or tensile) forces - tend to lenghten aspecimen

Compression forces - tend to shorten a specimen

Shear Forces - tend to cause parallel sections of a specimen to slide

Torsion Forces - tend to twist a specmen

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Decimal and Exponent Formats

Decimal Scientific Notation

0.000001 / 1x10^-6

0.001 / 1x10^-3

0.01 / 1x10^-2

0.1 / 1x10^-1

1 / 1x10^0

10 / 1x10^1

100/ 1x10^2

1000000 / 1x10^6

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Unit Prefixes and Multipliers

Multipliers Unit Prefix Symbol

10^-12 / pico / p

10^-9 / nano / n

10^-6 / micro /m

10^-3 / milli / m

1 / - / -

10^3 / kilo / k

10^6 / mega / M

10^9 / giga / G

10^12 / tera/ T

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Stress is applied force per unit area

  • Stress= Force Units= Nm-2 or Pa
  • Area

Yield Stress is the value of force at which a material becomes permanently deformed.

This occurs at the elastic limit or yield point

Yield Stress is the limit of a materials useful strength

Breaking Stress (or ultimate tensile strength) is the value of force at which the material breaks

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Strain is the fractional change of length caused by an applied stress

  • Strain = Change of Length
  • Original Length

Strain may be expressed as:

  • A Decimal = 0.5
  • A Fraction = 1/5
  • A Percentage = 20%
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Young's Modulus

Young's Modulus is a measure of the stiffness of a material

  • Youngs Modulus = Stress
  • Strain

The larger the young's modulus value, the stiffer the material, so a larger stress is required to produce a given strain.

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Hooke's Law

when tensile tested, good steel springs produce a simple force-extension graph that has an initial straight line portion.

this indicated that, Force, F µ

When tensile tested, good steel springs produce a simple force-extension graph that has an initial straight line portion This indicates that Force, F µ x, extension and constant of proportionality, k = F x k is called the stiffness of the specimen and its units are Nm-1

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